NMR investigation of selected interior regions of an object have long employed a surface coil disposed on the exterior surface of the object proximate a region of interest to localize an RF resonance signal incident to magnetic resonance excitation. A surface coil has the attributes of an antenna and is desirably of no significantly greater extent than the region of interest. The surface coil is often analogized to a loop antenna in combination with a tunable resonant circuit. The ideal surface coil provides strong coupling to a delimited region of the object with enhanced sensitivity and minimal radiation losses.
In typical imaging instrumentation and usage, excitation of nuclear resonance is effected with a volume coil providing excitation of resonance over a volume region of the object under study. Local excitation may be obtained with a surface coil, although it is rather more common practice to employ the surface coil in the receive channel. This is by no means a limitation in respect to the present invention.
Certain surface coils for NMR use have been inspired by the use of ladder circuits to produce particularly effective coupling between the nuclear spins of the object and an RF source or sink. See U.S. Pat. Nos. 5,898,306; 6,169,401. Ladder circuits are the usual analytic model for transmission lines, which are often modeled as LC ladder circuits in consideration of the inductance of two spaced apart conductors furnishing distributed inductance and the distributed capacitance therebetween. The ladder circuit is conveniently analyzed as a four terminal network/device. This network exhibits inductive and capacitive reactances distributed over the network. The ladder network is commonly periodic in a basic mesh, or loop: that is, there is discerned an axis of periodicity over which the elementary mesh units repeat, forming the ladder. Ladder networks have been employed in prior art but it will be apparent that the construction of any ladder network/surface coil requires careful selection of discrete circuit components to preserve the design integrity of the circuit as well as laborious manufacture of the coil from a variety of components. As inductance and capacitance become continuously distributed, the network acquires the functional aspect of a transmission line. In conventional usage, an RF current is applied to the terminals at one (axial) end of the transmission line/ladder network propagates along the axis of periodicity to a specified load or short circuit connected to the other (axial) end terminals. The transmission line/ladder network acquires the character of a tuned circuit when the axial length of the line/network is nλ/2 where n is an integer. In an example of prior art of this form, surface coils have been implemented from coaxial transmission line components in prior art. In one such arrangement a loop is formed in coaxial cable and the free end of the inner conductor is shorted to the free end of outer conductor, this shorted end being connected to the standing portion of the outer conductor to form the loop. Opposite this connection point, or vertex (e.g., 180° therefrom) of the loop, the outer conductor is interrupted to form a small gap. Thus, outer conductors on both sides of the gap are electrically joined opposite the gap, and the free end of the inner conductor is similarly shorted to the outer conductor at the vertex of the loop. The remaining coaxial conductor length leading away from the loop communicates with an amplifier. In this prior art, the full line length from the amplifier around the loop to the vertex is nλ/2. See U.S. Pat. No. 4,816,766. The dimensional constraint on surface coils is undesirable.